In recent times, wireless handsets and terminals have evolved to have a high level of functionality while also becoming extremely compact. Wireless handsets and terminals often include a range of personal media functions, and are capable of operating on multiple systems such as the Global System for Mobile Communications (GSM) and the Universal Mobile Telephone System (UMTS). The components of the various systems in a contemporary wireless handset are required to offer high performance while the physical dimensions are required to become progressively smaller.
In the RF front-end circuit of a wireless handset, a power monitoring circuit is usually employed to control the transmitted power, for example, to ensure that the handset conforms with emission regulations pertaining to the system of operation and in the region of operation and in order to conserve battery life. A prior art block diagram of a conventional power monitoring circuit of an RF front-end circuit is shown in FIG. 1.
The directional coupler is a well known RF device which is used for monitoring the level of power traveling along a signal line in a particular direction. A directional coupler comprises a pair of transmission lines which are in close physical proximity to each other so that they become electromagnetically coupled to each other. A single transmission line can be characterized primarily by its electrical length and its characteristic impedance, thus a pair of transmission lines has a pair of electrical lengths and a pair of characteristic impedances. A coupled pair of transmission lines, such as those of a directional coupler, are more commonly characterized by the even mode impedance and the odd mode impedance and the even mode phase length and the odd mode phase length of the coupled transmission lines.
FIG. 2A shows a diagram of a prior art directional coupler. The directional coupler of FIG. 2A comprises a pair of transmission lines 25 which are electromagnetically coupled to each other. Both of the transmission lines have input/output (I/O) ports at each end, so that the pair of coupled transmission lines 25 comprise four I/O ports. The I/O ports are labeled as an input port 21, a direct port 22, a coupled port 23 and an isolated port 24. The pair of transmission lines of the directional coupler of FIG. 2A are formed so as to be embedded inside or on the surface of an insulating substrate and the transmission lines may be arranged to provide broadside coupling i.e. where respective broadsides of each line are adjacent to each other or to provide edge coupling i.e. where respective edges of each line are adjacent to each other. When a signal is fed to the input port 21 of the directional coupler of FIG. 2A, inevitably, some of the signal is fed to the output port 22; however, the electromagnetic coupling between the transmission lines is such that a signal on one line induces a corresponding signal on the other line so that some of the input signal is also fed to the coupled port 23, and under certain (non-ideal) conditions some of the input signal may also be fed to the isolated port 24.
The structure depicted in FIG. 2A has at least one axis of symmetry 20, and may have further axes of symmetry (not shown), so the designation of labels to the ports is somewhat arbitrary; for example, an input could be fed to port 22, so that the direct port would become port 21, the coupled port would become port 24 and so that the isolated port would become port 23.
Directional couplers can be broadly categorized as either equal coupling or weakly coupled. Directional couplers offering roughly equal power splitting between the direct port and the coupled port—known as 3 dB couplers—typically comprise transmission lines having an electrical length equal to one quarter of the wavelength of the operating frequency of the coupler. Weakly coupled directional couplers, i.e. those which pass most of the input power to the direct port, and which couple only a small percentage thereof to the coupled port, may also comprise lines with an electrical length equal to one quarter of one wavelength; alternatively, such couplers can be fabricated using lines which are much shorter than one quarter of one wavelength. The choice of the electrical length depends on the required operating bandwidth, the required coupling ratio and the physical limitations of the fabrication process.
For couplers comprising short transmission lines (i.e. where the electrical length of the transmission lines is substantially less than one quarter of one wavelength at the frequency of operation of the directional coupler) and lines of equal length, the even mode phase length and odd mode phase length are approximately equal. Hence, such couplers can be characterized by three main parameters: the even mode impedance, the odd mode impedance, and the electrical length.
The operating performance of a directional coupler is usually given in terms of four electrical specifications: the coupling ratio, the insertion loss, the isolation and the return loss. These specifications can be determined analytically from the characterizing parameters of the directional coupler, or by direct measurement. The first specification, the coupling ratio, is a measure of the RF power which is emitted at the coupled port for a given level of power fed to the input port. Typically, this value is expressed as a ratio measured in decibels. Practical coupling ratios can vary from as low as −40 dB (corresponding to very weakly coupled lines) to −3 dB (strongly coupled lines providing equal power splitting between the direct port and the coupled port). The second specification for the performance of a directional coupler is the insertion loss for signals passing between the input port and the direct port. For couplers offering weak coupling between the input port and the coupled port, the insertion loss should be very low; for example, a coupling ratio of 1:10 (−10 dB at the coupled port) will give rise to a theoretical minimum insertion loss of 0.45 dB. Table 1 gives the relationship between the coupling ratios (in decibels) and the minimum insertion loss for a matched RF coupler. The third specification of the directional coupler is the isolation. A well designed directional coupler will feed power from the input port to the direct port and to the coupled port only. Thus, there should be no power at the isolated port so that an ideal coupler would have infinite isolation. In practice, some power is always passed to the isolated port, and the isolation of the coupler gives the relative level of this power. The final specification of a directional coupler, the return loss, can be measured at each port. Typically, a directional coupler is designed to be terminated into 50Ω loads at each port, and the return loss is a measure of how closely matched the impedance presented by the coupler at a given port is to the impedance terminating the same port.
An alternative measure of the isolation of a directional coupler is the directivity, which is the isolation in decibels minus the coupling ratio in decibels. In this context, a coupler can be described as a high directivity coupler if there is a very low ratio of the power fed to the isolated port from the input port compared with the power fed to the coupled port from the input port.
It is well known in the design of a directional coupler, that a critical requirement for high isolation and high directivity is that the product of the even mode impedance Z0E of the coupled transmission lines with the odd mode impedance Z0O of the coupled transmission lines should be equal to the square of the reference terminating impedance Z0 on the four ports of the directional coupler—see EQUATION 1 below. For example, see Mongia, R; Bahl, I; Bhartia, P; “RF and Microwave Coupled Line Circuits” ISBN: 0-89006-830-5; Artech House 1999; pp 137. The standard reference impedance Z0 in most RF applications is 50 Ohms.Z0O×Z0E=ZO2  EQUATION 1
Generally speaking, the even mode impedance is determined by the physical dimensions of the coupled transmission lines the properties of the material surrounding them and the proximity of the coupled transmission lines to RF ground. On the other hand, the odd mode impedance is a function of the physical dimensions of the coupled transmission lines the properties of the material between the two transmission lines and the proximity of the coupled transmission lines to each other. Thus, both parameters are independent of each other, and the criteria of EQUATION 1 can be met provided that there are no limitations in the fabrication process of the coupled transmission lines.
FIG. 2B shows an alternative prior art directional coupler which includes resistive attenuators 26, 28 connected at the coupled and isolated ports of FIG. 2A respectively. Resistive attenuators 26, 28 are both two terminal devices, a first terminal of resistive attenuator 26 is connected to coupled port 23, and a second terminal of attenuator 26 provides a matched coupled port 27 of the directional coupler; similarly, a first terminal of resistive attenuator 28 is connected to isolated port 24, and a second terminal of attenuator 28 provides a matched isolated port 28 of the directional coupler. Resistive attenuator 26, connected at coupled port 23, is provided to reduce the effect of a mismatch from a connection at matched coupled port 27 of the directional coupler. A mismatch would occur, for example, if the impedance connected at matched coupled port 27 of the directional coupler were not exactly equal to 50 Ohms, and in typical applications, this can often be the case. As an example, a 5 dB attenuator connected at coupled port 23 would improve the return loss at matched coupled port 27 of the directional coupler by 10 dB. Conversely, the use of an attenuator in the manner shown in FIG. 2B ensures that, regardless of the termination at matched coupled port 27, the impedance presented to the coupled pair of transmission lines 25 is close to the required reference impedance and thus that the conditions of EQUATION 1 are met.
The attenuator 28 at the isolated port 24 of FIG. 2B is provided for symmetry, i.e. if the directional coupler is to be used in reverse, with power being fed to direct port 22 and power being coupled to isolated port 24. The attenuator 28 at isolated port 24 will minimize the effect of any mismatch which may be connected at isolated port 24. Attenuators 26, 28 do not significantly affect the insertion loss of the directional coupler of FIG. 2B. Attenuator 26 gives rise to a reduction in the coupling ratio; however, compensation for this effect is possible by re-design of the pair of coupled transmission lines for higher coupling. As an example, it can be seen from TABLE 1 that for directional couplers providing coupling ratios of less than −15 dB, compensation for the addition of a 5 dB attenuator at the coupled port will produce a degradation of 0.32 dB or less in the insertion loss of the directional coupler.
TABLE 1Theoretical Minimum Insertion Loss of a Directional Coupler for a givenCoupling Ratio.Percentage of Input PowerRelative Power fedTheoretical Minimumfed to Coupled Portto Coupled Port/dBInsertion Loss50%−3.0 dB  −3.0 dB25%−6.0 dB −1.25 dB10%−10 dB−0.46 dB 3%−15 dB−0.14 dB 1%−20 dB−0.04 dB0.3% −25 dB−0.01 dB
FIG. 3 shows a block diagram of part of the TX section of a prior art RF front-end circuit which includes a directional coupler and other components to monitor power levels emitted from the power amplification stage and to monitor power levels reflected from the antenna. A percentage of the RF power emitted by the power amplifier (PA) is fed via the directional coupler to the first power detector so that the level of power emitted by the PA can be monitored. Similarly, a percentage of the RF power reflected back into the circuit by the antenna is fed via the directional coupler to the second power detector. Hence, the directional coupler in the circuit of FIG. 3 facilitates independent monitoring of the RF power emitted by the PA and the RF power reflected by the antenna. However, independent monitoring of these two power levels requires that the isolation of the directional coupler is sufficiently high to prevent a significant percentage of the signal emitted from the PA being fed directly to the 2nd power detector. Specifically, for a capability to measure two substantially different power levels emitted by the PA and reflected by the antenna (say a difference of 20 dB), the directional coupler is typically required to have a very high directivity E.G. 25 dB or higher.
From the description of the prior art provided above, it is clear that for RF power monitoring applications, a directional coupler is required to be compact, and to offer high directivity.
Significant problems in the design and fabrication of directional couplers arise from the limitations in the accuracy and control over the fabrication of transmission lines with the required physical dimensions. Similar problems arise due to the limitations in the consistency of the material properties of the substrate on which the transmission lines are fabricated and batch variations in the thickness of the substrate. These limitations influence the capability to fabricate a coupler which meets the conditions of EQUATION 1. Furthermore, in the design of a directional coupler, the choice of available substrates is also limited to a few materials and a few discrete substrate thicknesses.
The drive for greater miniaturization is another limiting factor: the realization of a directional coupler with sufficiently small outer dimensions typically demands transmission lines that have physical dimensions which may be outside the capability of the fabrication process. For example, fabrication of a directional coupler on a thin substrate allows a reduction in the height of the coupler, and the use of a substrate with a high dielectric constant allows for reduction in the length of the coupled transmission lines of the coupler for a given coupling ratio. However, the use of a thin substrate will lower the even mode impedance of the coupled transmission lines, and the use of a substrate with a high dielectric constant will lower both the even mode impedance and the odd mode impedances of the coupled lines.
It is possible to compensate for the reduction in the even mode impedance by using narrower transmission lines; however the design rules of the production process typically sets a lower limit on the dimensions of lines. On the other hand, it is possible to compensate for a low odd mode impedance arising from the use of a substrate with a high dielectric constant by designing a coupler with transmission lines which are spaced further apart; unfortunately, increasing the spacing between the transmission lines lowers the coupling ratio of the directional coupler, and the only way to compensate for a lower coupling ratio is to use longer transmission lines thereby canceling any the benefit of selecting a high dielectric substrate for miniaturization.
In summary, the designer of a miniaturized directional coupler is faced with the dilemma that dimensions of the coupled transmission lines, and the electrical properties of the material of the substrate determine the even mode impedance and the odd mode impedance of the directional coupler, but that the product of the even mode impedance and the odd mode impedance of the directional coupler must equal the square of the reference impedance according to EQUATION 1—2500Ω2 for conventional RF applications. Hence, the designer is presented with a limited range of options to produce a directional coupler of the required size with the required performance and which can be fabricated to the required precision.
To overcome these problems, the designer needs an additional degree of freedom when selecting line widths and line spacing for producing a miniaturised directional coupler.
As mentioned previously, it has been well established in the design of a directional coupler, where high directivity is a goal, that the product of the even mode impedance and the odd mode impedance should be equal to the square of the reference impedance—see EQUATION 1. This condition, while valid, does not provide the most general requirement.
Referring once again to FIG. 2A, the most general requirement for the design of a directional coupler with high directivity is that the product of the impedance terminating the direct port 22 and the impedance terminating the coupled port 23 should be equal to the product of the even mode impedance Z0E and the odd mode impedance Z0O of the coupled transmission lines. This relationship is given by EQUATION 2ZP2×ZP3=ZOO×ZOE  EQUATION 2where ZP2 is the value of the impedance terminating the direct port 22 and where ZP3 is the value of the impedance terminating the coupled port 23.
In practical use, the impedance terminating the direct port of a directional coupler ZP2 will invariably be the reference impedance. In fact, the assumption that the reference impedance terminates all ports of a directional coupler is the starting point in most technical analyses on the subject. However, it is possible to transform the impedance terminating the coupled port using an impedance transformation circuit. One example of a circuit which can provide impedance transformation is a resistive attenuator, such as a PI-type resistive attenuator. Conveniently, as described above and as illustrated in FIG. 2B a resistive attenuator can be used advantageously in a directional coupler to provide matching of a poor or unknown termination at the coupled port. However, a resistive attenuator may also be used to provide impedance transformation of a reference impedance to some other value.
FIG. 4 shows an exemplary drawing of a prior art PI-type attenuator circuit which can provide both impedance matching and attenuation and which can also provide impedance transformation. The level of attenuation and the impedance transformation ratio of the circuit of FIG. 4 is determined by the values of resistors R41, R42, and R43.